public IReadOnlyList<NetworkDiagram> GetNetworkDiagrams( 
   Envelope extentOfInterest,
   IEnumerable<Guid> globalIDs
)

{
  static void GetDiagram(UtilityNetwork utilityNetwork, IEnumerable<Guid> globalIDs)
  {
    Envelope extentOfInterest = MapView.Active.Map.GetDefaultExtent();

    using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
    {
      // Get all the diagrams
      IReadOnlyList<NetworkDiagram> diagrams = diagramManager.GetNetworkDiagrams();

      // Get a diagram by name
      NetworkDiagram diagram = diagramManager.GetNetworkDiagram(_diagramName);

      // Get diagrams by extent
      diagrams = diagramManager.GetNetworkDiagrams(extentOfInterest);

      // Get diagrams from a set of utility network feature GlobalIDs
      diagrams = diagramManager.GetNetworkDiagrams(globalIDs);

      // Get diagrams from a set of utility network feature GlobalIDs within an extent
      diagrams = diagramManager.GetNetworkDiagrams(extentOfInterest, globalIDs);
    }
  }
}

{
  static List<NetworkDiagram> GetInconsistentDiagrams(UtilityNetwork utilityNetwork)
  {
    // Get the DiagramManager from the utility network
    using (DiagramManager diagramManager = utilityNetwork.GetDiagramManager())
    {
      List<NetworkDiagram> networkDiagrams = new List<NetworkDiagram>();

      // Loop through the network diagrams in the diagram manager
      foreach (NetworkDiagram diagram in diagramManager.GetNetworkDiagrams())
      {
        NetworkDiagramInfo diagramInfo = diagram.GetDiagramInfo();

        // If the diagram is not a system diagram and is in an inconsistent state, add it to our list
        if (!diagramInfo.IsSystem &&
            diagram.GetConsistencyState() != NetworkDiagramConsistencyState.DiagramIsConsistent)
        {
          networkDiagrams.Add(diagram);
        }
        else
        {
          diagram.Dispose(); // If we are not returning it we need to Dispose it
        }
      }

      return networkDiagrams;
    }
  }
}